Superconductive alloys



Aug. 23, 1966 Filed May 2l, 1965 W. T. REYNOLDS SUPERCONDUCTIVE ALLOYS 4Sheets-Sheet l Aug- 23, 1966 w. T. REYNOLDS SUPERCONDUCTIVE ALLOYS 4Sheets-Sheet 2 Filed May 2l, 1965 @EL @mi 2225; o Emma 2223; o tm o9 omo@ o v oN o OQ om o@ o om o O O O O oN l No om o@ 1 o w m ov a u w m llxs U.. m vw w v owl wow 1 om d l S lun |n m .n 30d .fr lq. O l W 93 iOmmc@ @.oum .on om l AW: ud H V H m l l V3 .v dm N c oo r Q u o9 o u1-G xam 7H M H1 A ONT mw 0N. J o2 l $525 o2 mom 0 1 352:). o9 mom I l DooomS2 oooom S2 L Aug- 23, 1966 w. T. REYNOLDS 3,268,373

SUPERCONDUCTIVE ALLOYS Filed May 2l, 1963 4 Sheets-Sheet 3 AGED AT 400CFOR |50 MINUTES WEIGHT TITANIUM Aug. 23, 1966 Filed May 2l, 1963 W. T.REYNOLDS SUPERCONDUCTIVE ALLOYS 4 Sheets-Sheet 4 N217 .LV 'OH'G'IEIH'IVOILIHO United States Patent O M 3,268,373 SUPERCNDUCTWE ALLOYSWilliam T. Reynolds, Peters Township, Washington Connty, Pa., assignorto Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation ofPennsylvania Filed May 21, 1963, Ser. No. 282,035 13 Claims. (Cl.14S-32.5)

This invention is directed to a process `for making superconductivealloy strip or wire having a high critical iield and a high criticalsupercurrent density in a strong applied magnetic iield and to thesuperconductive alloy strip or wire made thereby.

The phenomenon of superconductivity at cryogenic temperatures has beenyknown for many years, but it is only recently that practicalapplication of the phenomenon has become feasible. One such applicationis the fabrication -of electromagnetic coils or soleno-ids fromsuperconductive wire or strip for'the development of high magnetichelds. A substantial degree of success has already been achieved withsuch electromagnetic coils and magnetic elds in excess of 50,000 gausshave been developed.

4In producing superconductive magnetic coils, the phenomenon ofsuperconductivity in certain metals and alloys is relied upon. Briefly,as a coil of the wire of the metal is cooled there is reached a pointusually within several degrees of absolute zero (such point beingspeciic to the particular metal or alloy, and known as the criticaltemperature) -at which the metal loses its normal resistance to the ilowof electrical current and a small electrical current will ilow in thecoil more or less indenitely, -and the metal is in what is called thesuperconducting state. 'Iihis property of superconductivity ismaintained at temperatures below the critical temperature and disappearsabove the critical temperature. The amount of electrical current thatthe conductor can carry in the superconductive state has -a maximum,known as the critical supercurrent density-Jc, which if exceeded causes.t-he conductor to lose its superconducting properties. Further, a wireor coil in the superconductive state is atected by a magnetic heldeither self-induced `or externally applied, which if of Ih-igh enoughdensity rwill cause the conductor to lose its supercond-uctiveproperties, such magnetic iield being designated the critical -`1eld-Hc.At magnetic lields of values less th-an the critical Iiield, theconductor can carry only a certain maximum supercurrent density, themaximum supercurrent density increasing with lower magnetic flux density-on the conductor.

Many of the electromagnetic coils of high quality which have been madehave been wound from niobiumzirconium alloy wire. This niobium-zirconiumalloy wire must ybe hot worked initially and yannealed at least once inthe course of cold working to maintain the material in a workablecondition. Further, nio-bium-zirconium wire employs a relatively highproportion of niobium therein and is consequently quite expensive.

The niobium-titanium `alloy system is a superconductive alloy system buthas not been extensively investigated. 'Ibis alloy system possessesseveral characteristics which are attractive for superconductorapplications. First, the critical temperature is above 9.0 K. for binaryniobium-titanium alloys containing from to approximately 60 atom percenttitanium. Second, the resistive critical eld at 4.2 K. is approximately120 kilogauss. Third, titanium is -a relatively cheap and an abundantalloy component.

Accordingly, it is the object of the invention to provide asuperconductive alloy conductor having a high critical iield and a highcritical supercurrent density in 3,268,373 Patented August 23, V1966 ICCstrong applied magnetic fields of a magnitude approaching .that of thecritical held.

It is la further object of the invention to provide a process for heattreating niobi-um-titanium superconductive alloys to increase thecritical field and critical supercurrent densities thereof.

Other objects and advantages of the invention will, in part, be obviousyand will, in part, appear hereinafter.

For a better understanding of the nature and objects of this invention,reference should be had to the following detailed description and to thedrawings, in which:

FIGURE 1 is a graph in which the critical eld and .the criticalsupercurrent density in a eld of 20 kilogauss for an as-rolledniobium-titanium alloy -are plotted against the titanium content of thealloy;

FIG. 2 is a grap-l1= simular to that of FIG. l in whichniobiurn-titanium alloys have been cold rolled, aged at room temperatureand heat treated at 200 C. `for 21/2 hours;

FIG. 3 is -a 'graph simi-lar to that of FIG. 1 in which theniobium-titanium .alloys have been cold rolled, aged at room temperatureand heat treated at 300 C. for 21/2 hours;

FIG. 4 is a graph similar to that of FIG. 1 in which theniobium-titanium alloys have been cold rolled, aged at room temperatureIand heat treated at 400 C. for 21/2 hours;

FIG. 5 is a graph similar to that of FIG. 1 in which theniobium-titanium alloys have been cold rolled, aged at room temperatureand heat treated at 500 C. for 21/2 hours;

FIG. 6 is a graph in which critical supercurrent density is plottedagainst the degree `of cold work in an N-b-l9.83% titanium alloy; and

FIG. 7 is a graph `similar to that of FIG. l in which theniobi-um-titanium alloys have been cold worked and aged at roomtemperature.

In accordance with this invention, the niobium-titanium superconductoralloys are subjected to 4a treatment at a moderately elevatedtemperature above 100 C. for a relatively short period of time. It hasbeen found that this heat treatment is of a substantial benelit only tothose alloys containing over 10% titanium.

The invention broadly comprises a superconductive alloy conductorcomposed `of from about 10% to about by weight of titanium, and thebalance niobium except tor small amounts of impurities, the alloyconductor having been subjected to a cold reduction of at least 96% toproduce a wire or strip therefrom and thereafter heat treated attemper-atures of C. and higher for at least 0.1 hour, and preferablylonger to provide that the conductor is characterized by a relatively'.high critical eld -under superconductive conditions and possessesimproved critical supercurrent density in applied magnetic iieldsapproaching the magnitude of the critical eld.

More specically, superconductive conductors of the invention comprise analloy which has been subjected to a cold reduction of at least 99% toproduce a thin strip or wire conductor, the conductor exhibiting undersuperconductive conditions a relatively high supercurrent density in anapplied magnetic field of 20 kilogauss, the alloy conductor lcomposed offrom about 10% to about 75% by weight of titanium and the balanceniobium except for small amounts of impurities, the alloy conductorhaving been heat treated for from about 0.1 hour to about 5 hours attemperatures in the range of from about 100 C. to 550 C.

A preferred superconductive conductor of the invention comprises analloy which has been subjected to a cold reduction of at least 99% toproduce a thin wire or strip, the conductor exhibiting undersuperconductive aaeaars conditions a critical field of at least 50kilogauss and a critical supercurrent density of at least about 0.4 10amps/ cm.2 in a magnetic field of 20 kilogauss, the alloy conductorcomposed of from about 10% to about 75%, by weight, of titanium, and thebalance niobium except for small amounts of impurities, the lalloyconductor having been heat treated for from about 0.1 hour to about 5hours at a temperature of about 200 C.

Still another preferred superconductive conductor comprises an alloywhich has been subjected to a cold reduction of at least 99%, theconductor exhibiting under superconductive conditions a critical fieldof at least 70 kilogauss and a critical supercurrent density of at leastabout 0.7 105 amps/ cm.2 in an applied magnetic field of 20 kilogauss,the alloy conductor composed of from about to about 75% by weight oftitanium and the balance niobium except for small amounts of impurities,the alloy conductor having been heat treated at a temperature of about400 C. for from 0.1 to 5 hours.

A heat treatment at temperatures in the range from 200 C. to 550 C. isextremely beneficial when applied to highly cold reduced alloy strip orwire containing at least 10% titanium. At heat treatment temperatures inexcess'of 600 C. the beneficial effect of heat treatment is slight. At700 C. the beneficial effect has essentially disappeared. In general,600 C. is a practical maximum for heat treatment.

An investigation of the niobium-titanium alloy system revealed thatmembers of these alloys in the cold worked condition have low criticalsuper-current density in high magnetic fields. Supercurrent densityvalues for a number of alloys have been determined as follows:

TABLE I Cold work Field applied Critical super- Cornposition (wt.percent reducparallel to current density percent) tion in area rollingdirecat 4.2 K.

tion (lrilogauss) (amp. l cnil) Nb-18.1 Ti- 96. 4 19. 5 15,850 Nb-26-0Ti- 96. 4 19. 0 4, 530 bib-9.88 Ti- 99. 428 20.0 1S, 039 Nia-19.83 Ti99. 423 20. 0 8, 279 N13-40.50 Ti- 99. 374 20. 0 3, 310 N13-57.59 Ti 99.413 20. 0 7, 640 N13-80.36 Ti 99. 428 20. 0 0

A 21/2 inch diameter of a niobium-60% titanium alloy with small amountsof incidental impurities is prepared by vacuum arc melting a compositeelectrode composed of 40 parts by weight of electron beam melted niobium(99.90% niobium) and 60 parts by weight of arc-melted -titanium (99.35%titanium). The vacuum arc-melted ingot is machined to remove surfaceroughness and imperfections. The ingot is then homogenized in thetemperature range 900-1400 C. in a vacuum of less than 10-4 mm. Hg for aperiod of several hours. The ingot is then cold forged to a slab havingthe dimensions 1" x 1" x 3". After the slab is surface conditioned bymachining, it is then cold rolled to 0.03 thick strip with reductions of10% to 20% per pass. The 0.03 strip is then cold rolled in a four-highrolling mill with reductions of 5% to 10% per pass to a final thicknessof .003". The strip of final thickness is then heat treated at 400 C.for 21/2 hou-rs in a vacuum annealing furnace.

Similarly to this example, a series of niobium-titanium alloy stripswere prepared with from 2% to 94% by weight of titanium. The results ofcryogenic testing of the niobiurn-titanium alloy strips of thisinvention so prepared are set forth in the FIGS. 1 through 6. FIG. 1which is derived from the result of tests on niobium-titanium alloystrips in essentially the as-rolled condition shows that thesupercurrent density Jc of the alloys in a 20 kilogauss field attainsdesirable values at titanium contents of Well below 20% and particularlybelow 10%. The critical eld Hc in the same composition range isrelatively poor and does not attain usefully high levels and is goodonly when titanium is at least about 20%. It should be noted that thecritical supercurrent density falls off rapidly as the titanium contentapproaches 30% while at the same time the critical field is attaining adesirable high level.

FIG. 2 is directed to the niobium-titanium alloy strips which have been`cold rolled, aged at room temperature about four months and heattreated at 200 C. for 21/2 hours. There is a phenomenal change in theproperties of the alloys as compared to FIG. l. Particularly, it shouldbe noted that the critical field and the critical supercurrent densityare at relatively high levels in alloys containing 10% or more titaniumand achieve maximum values in the broad composition range of from 10% to60% titanium. The critical field attains the value of about 126kilogauss.

FIG. 3 is directed to the niobium-titanium alloy strips which have beencold rolled, aged at room temperature about five months and heat4treated .at 300 C. for 21/2 hours. The supercurrent density achieves amaximum of about 0.9 105 amps/cm.2 in alloys containing about 40%titanium and the supercurrent density is over 0.7 105 amps/cm.2 fromabout 30% to over 65% titanium.

FIG. 4 is directed to the niobiurn-titanium alloy strips which have beencold rolled, `aged at room temperature about tive months and heattreated at 400 C. for 21A. hours. The drastic increase which hasoccurred in the critical supercurrent density curve is of particularinterest. It should be noted that the curve attains an extremely highmaximum and is at a level of over 1.0 105 amps/cm.2 in the applied fieldof 20 kilogauss over the composition range from 20% -titanium to overabout 70% titanium. Surprisingly, the critical field is alsosubstantially increased by this heat treatment in the composition range50% to about 80% titanium. Exceedingly good critical field values appearover the range of 20% titanium to about titanium.

FIG. 5 is directed to the niobium-titanium alloy strips which have beencold rolled, aged at room temperature about five months, and heattreated at 500 C. `for 21/2 hours. It is noted that in niobium-titaniumalloys containing less than 30%, the critical supercurrent density in afield of 20 kilogauss achieves a maximum at about 20% titanium. Between20% titanium and 40% titanium critical supercurrent density decreasesand critical field increases as titanium content 4is increased. `Optimumproperties in alloys con-taining more than 40% titanium are limited to arelatively narrow composition range of from 45% to 70% ti-tanium.

It has been found that the amount of cold reduction greatly affects thecritical supercurrent density of the alloy conductors of the invention.At least 96% cold reduction must be employed, and greater reductionswill produce better alloy members. in fact, reductions in excess of 99%are desirable and improvement continues as. the degree of cold reductionreaches and exceeds 99.99%. Typical results obtained with increasingamounts of cold Work are shown in FIG. 6.

In some cases, the improvement achieved by 'cold working alone issufficient to render the alloy superconductors of this invention usefulin superconductive magnets Witlh out further heat treatment. Thus, a 50%titanium-niobium alloy Wire of 0.005" diameter, which has undergone areduction in area of 99.9996% exhibited a supercurrent density of about1X 105 amps/cm.2 in an applied field of 20 kilogauss. Moderateimprovement in this property would be expected in cold worked wireplaced in serv-ice as the amount of room temperature aging accumulates.However, heat treatment above 100 C. to 550 C. will further improve thesuperconductive properties of the w-ire to give a total of 50% increasein Je.

It has been found that definite improvement in the supercurrentdensities of the cold worked alloys of this invention can be eected byprolonged aging at approximately room temperature (see FIG. 7). The termroom temperature is intended to include temperatures in the range ofabout 10 C. to 50 C. For even small improvement in properties -at leastthirty days of room temperature aging is required. `Continued slowimprovement in properties is observed up -to a year and more of aging atroom temperature. However, maximum current densities cannot be attainedby aging at room temperature.

Generally speaking, highest proper-ties are attained in alloys aged forseveral months at room temperature and then heat treated at the elevatedtemperature. It is clear that a technique calling for several months ofaging at room temperature is hardly a commercially desirable process.Further, the alloys of this invention have been aged at elevatedtemperatures promptly after -cold working and it has been found thatvery high current densi-ty values 4are attained, values entirelysatisfactory from the commercial point of view and approaching thevalues in FIGS. 2 to 5. It will 'be understood that a small amount ofaging at room temperature will occur in most cases due to the normaltime interval between cold working and aging at elevated temperature.This time interval at room temperature normally will not exceed two orthree weeks, and may amount to only a few hours or days. As has beenpointed out, such short times at room temperature will not significantlyaffect the supercurrent density. After aging at elevated temperature,-additional time at room temperature will not aiect the supercurrentdensity.

The niobium-titanium alloys in accordance with this invention arecomparable in their superconductive properties to the niobium-zirconiumalloys now in use. The alloys of the invention are relatively easy tocold work. The relatively large proportion of titanium characterizingmany of the alloys of this invention, results in a much less expensivesuperconductive material.

From the foregoing disclosure and data, it is evident that the presentinvention provides superconductive -materials having properties whichare highly useful in superconductive applications.

The inventive principles embodied in the above description may obviouslybe incorporated in modied processes by those skilled in the art withoutdeparting from the spirit and scope of this invention, and it isintended that the description be interpreted as illustrative and not ina limiting sense.

I claim as my invention:

1. A superconductive alloy conductor which has been subjected to a coldreduction of at least 96%, said conductor exhibiting undersuperconductive conditions a relatively high critical eld and improvedcritical supercurrent density in a strong applied magnetic eld, saidalloy conductor composed of from about 10% to 75% by weigh-t of titaniumand the balance niobium except for trace amounts of impurities, thealloy conductor having been heat treated for at least 0.1 hour `attemperatures in the range of 100 C. to 600 C.

2. A superconductive alloy conductor which has been subjected to a coldreduction of at least 99%, said conductor exhibiting undersuperconductive conditions a critical tield of at least 50 kilogauss anda lcritical supercurrent density of at least 0.4 105 amps/cm.2 in amagnetic field of kilogauss, said alloy conductor composed of 6 fromabout 10% to 75%, by weight, of titanium, and the balance niobium exceptfor trace .amounts of impurities, the alloy conductor having been heattreated for from about 0.1 hour to 5 hours at temperatures in the rangefrom C. to 550 C.

3. A superconductive alloy conductor which has been subjected to a coldreduction of at least 99%, said conductor exhibiting undersuperconductive conditions a critical eld of at least 50 kilogauss and acritical supercurrent density of at least 0.4X amps/cm.2 in a magneticeld of 20 kilogauss, said alloy .conductor composed of from about 10% to75 by Weight, of titanium, and the balance niobium except for traceamounts of impurities, the alloy conductor having been heat -treated forfrom about 0.1 hour to about 5 hours at a temperature of about 200 C.

4. The alloy conductor of claim 3 wherein after cold working theconductor is aged at approximately room temperature for at least onemonth prior to heat treatment at elevated temperature.

5. A superconductive alloy conductor which has been subjected to a coldreduction of at least 99%, said `conductor exhibiting undersuperconductive conditions a critical Iield of at least 70 kilogauss anda critical supercurrent density of at least 0.7 105 amps/ cm.2 in amagnetic tield of 20 kilogauss, said alloy conductor composed of fromabout 10% to 75 by weight, of titanium and the balance niobium exceptfor trace amounts of impurities, the alloy conductor having been heattreated for from about 0.1 hour to about 5 hours at a temperature ofabout 400 C.

6. The alloy conductor of claim 5 wherein after cold Working theconductor is aged at approximately room temperature for at least onemonth prior to heat treatment at elevated temperature.

7. A superconductive alloy conductor which has been subjected to a coldreduction of at least 99%, said conductor exhibiting undersuperconductive conditions a critical eld of at least 100 kilogauss anda critical supercurrent density of at least 1.0 105 amps/cm.2 in amagnetic field of 20 kilogauss, said alloy conductor composed of fromabout 20% -to 70%, by weight, of titanium, and the balance niobiumexcept for trace amounts of impurities the alloy conductor having beenhea-t treated for from about 0.1 hour to about 5 hours at a temperatureof about 400 C.

8. The alloy conductor of claim 7 wherein after cold Working theconductor is aged .at approximately room temperature for at least onemonth prior to heat treatment at elevated temperature.

9. A superconductive alloy conductor which has been subjected to a coldreduction of at least 99%, said conductor exhibiting undersuperconductive conditions a critical eld of at least 100 kilogauss anda critical supercurrent density of at least 1.5 105 amps/cm.2 in amagnetic eld of 20 kilogauss, said alloy conductor composed of about60%, by Weight, of titanium, and the balance niobium except for traceamounts of impurities, the alloy conductor having been heat treated forabout 21/2 hours at a temperature of about 400 C.

10. The alloy conductor of claim 9 wherein after cold Working theconductor is aged at approximately room temperature for at least onemonth prior to heat treatment at elevated temperature.

11. A superconductive alloy conductor which has been subjected to a coldreduction of at least 99%, said conductor exhibiting undersuperconductive conditions a critical lield of at least 70 kilogauss anda critical supercurrent density of at least 0.7 105 amps/ cm.2 in amagnetic eld of 20 kilogauss, said alloy conductor composed of fromabout 45% to 70% by weight of titanium and the balance niobium exceptfor trace amounts of impurities, the alloy conductor having been heattreated at a temperature of from 350 C. to 550 C. for from 2 to 4 hours.

12. The alloy conductor of claim 11 wherein after cold Working theconductor is aged at approximately room temperature for at least onemonth prior to heat treatment at elevated temperature.

13. A superconductive coil comprising a plurality of turns of a`conductor Comprising a highly cold reduced alloy consisting essentiallyof from about 10% to 75% by weight of titanium and the balance niobiumexcept for incidental impurities, the conductor having been heat 100 C.to 600 C.

References Cited bythe Examiner UNITED STATES PATENTS HiX 75-174 Jaiee75--1755 Berger 75--175.5 XR Matthias 75-175.5 XR

DAVID L. RECK, Prz'mmy Examiner.

treated at least .l hour at temperatures in the range of lo C. N.LOVELL, Assistant Examiner.

1. A SUPERCONDUCTIVE ALLOY CONDUCTOR WHICH HAS BEEN SUBJECTED TO A COLDREDUCTION OF AT LEAST 96%, SAID CONDUCTOR EXHIBITING UNDERSUPERCONDUCTIVE CONDITIONS A RELATIVELY HIGH CRITICAL FIELD AND IMPROVEDCRITICAL SUPERCURRENT DENSITY IN A STRONG APPLIED MAGNETIC FIELD, SAIDALLOY CONDUCTOR COMPOSED OF FROM ABOUT 10% TO 75% BY WEIGHT OF TITANIUMAND THE BALANCE NIOBIUM EXCEPT FOR TRACE AMOUNTS OF IMPURITIES, THEALLOY CONDUCTOR HAVING BEEN HEAT TREATED FOR AT LAST 0.1 HOUR ATTEMPERATURES IN THE RANGE OF 100*C. TO 600*C.